1. Field of the Invention
The present invention relates to an apparatus of the type used to hold an image-bearing photographic transparency in intimate contact with a photo-sensitive surface in which it is desired to register an image. More particularly, the invention relates to photographic exposure apparatus of the type which uses vacuum pressure to hold an image-bearing sheet in intimate contact with a photo-sensitive surface while exposing both to a light source
2. Description of Background Art
Photographic processes are used extensively in the printing industry to make printing plates, cylinders and the like which are then used to transfer ink images to the surface of paper, fabric, metal and other materials.
For example, metal plates used in offset printing are first coated with a photo-sensitive material. Next, a negative photographic transparency containing the printed words and pictoral content to be printed is placed in intimate contact with the photo-sensitized offset printing plate. The upper surface of the transparency is then exposed to a strong light source, causing photochemical reactions in various areas of the photo-sensitizied plates proportional to the transparency of the corresponding area of the transparency. Since the exposure process generally requires a substantial period of time, it is essential that the transparency and plate remain in a fixed position with respect to one another during the exposure period. Otherwise, the image produced in the exposed photo-sensitive material will be blurred.
The requirement for stable registration of an image on the photo-sensitive material necessitates the use of an apparatus which holds the photo-sensitive plate and the photographic transparency immobile during exposure. Vacuum pressure is a frequently used means by such apparatus to draw the transparency and plate tightly together during the exposure operation. This method of preventing movement of the transparency relative to the photo-sensitized plate has the additional advantage of minimizing the path length of light rays having passed through the transparency to the photo-sensitive surface. Minimum light path lengths are required in this form of exposure, referred to as a contact exposure, to ensure production of a sharp, well-defined image.
Several U.S. Pats. disclose exposure registry apparatus employing vacuum pressure. For example, Ott, in U.S. Pat. No. 3,066,592, Dec. 4, 1962, discloses the use of a flexible cord to define and seal the perimeter of a vacuum-tight region between a flexible acetate sheet and a rigid photo-sensitized plate. Oscarrson, in U.S. Pat. No. 4,032,233, June 28, 1977, and Jacobs, in U.S. Pat. No. 4,089,603, May 16, 1978, both disclose the use of a resilient gasket to form the perimeter of a vacuum-tight region between flat plates. Heitmann, in U.S. Pat. No. 4,423,851, Jan. 3, 1984, discloses the use of a rubber cloth with an integral, resilient bead to form a vacuum-tight seal with a flat plate.
After a photographic printing plate has been exposed as described above, the plate is treated with chemicals which develop the latent image present in the exposed photo-sensitive surface. The developing process makes those areas on the surface of the plate which are to transfer ink to a surface during the printing process selectively cohesive to ink.
In preparing a silk screen for printing by the silk screen process, a photographic method similar to that used to prepare photo offset printing plates is often used. In the customary method of placing an image to be printed on a silk screen, a photographic positive is placed directly on a silk screen which has been coated with a photo-sensitive emulsion, and both screen and positive are placed together in an exposure frame. Typically, the frame uses vacuum to hold the positive and screen tightly together. The positive and screen are then exposed to a bright light source which hardens the photo-sensitive emulsion in the exposed areas. The screen and positive are then removed from the frame and separated. In the final step of preparing the screen for printing, the screen is flushed with water. This operation selectively dissolves and removes the unexposed areas of the photo-sensitive emulsion.
The silk screen prepared as described above is used to print images on a surface by placing the lower surface of the screen in close contact with a surface, pouring ink on the upper surface of the screen above the image, and then drawing a squeegee across the ink. This forces ink through the open areas of the screen, and onto the surface to be printed.
As has been mentioned previously, the various types of apparatus used to hold a photographic positive in tight contact with a photo-sensitized surface of a printing plate or silk screen frequently employ vacuum pressure to clamp the two together during the exposure process. Many of the machines performing this function are costly, complicated, and limited in their flexibility to perform various clamping tasks related to photographic exposure. In response to a perceived need for an improved vacuum exposure registration apparatus, the present invention was conceived of.